A. Field of the Invention
This invention relates in general to inverter driven centrifugal liquid chillers and more specifically, to the control system for controlling the capacity of an inverter driven centrifugal compressor.
More specifically, without restriction to the particular use which is shown and described, this invention relates to an adjustable surge and capacity control circuit for an inverter driven centrifugal compressor based liquid chiller whereby the control circuit of the invention permits a capacity control system to be matched to existing equipment having unknown surge characteristics.
In the operation of liquid chillers employing inverter driven centrifugal compressors, it is advantageous for efficient and economical operation to regulate the capacity of the system by adjusting both the position of inlet guide vanes (PRV) and the compressor motor speed. In such chillers it is desirable that the motor speed and PRV position adjustments be made over an extended range without compressor surge occurring. Surge becomes a significant problem when the pre-rotation vanes (PRV) are in a condition nearly closed whereby small changes in vane position has a very substantial effect on the compressor capacity and can send the compressor into detrimental surging.
One capacity control system, which is highly effective in controlling capacity through adjustment of PRV position and compressor motor speed, while avoiding surging, is disclosed in U.S. Pat. No. 4,151,725 to kountz et al and assigned to the assignee of the present invention. In the control system disclosed in U.S. Pat. No. 4,151,725, circuitry is provided which develops the set point compressor speed as a function of pre-rotation vane (PRV) position. By regulating the speed of the compressor and the extent of opening of PRV in a manner to follow the mathematical function developed for a sytem having known characteristics, a control system, which avoids surge and achieves highly efficient operation, is disclosed in the subject patent to Kountz et al.
Although the circuit described in U.S. Pat. No. 4,151,725 is capable of attaining efficient operation of a chiller, the control system therein is not readily adaptable to control an inverter driven centrifugal liquid chiller having unknown surge characteristics. Thus, a factory set function, which is described in the Kountz patent, does not provide on-site flexibility in input/output relationship. Without such adaptability, it is possible that a centrifugal chiller having undetermined surge characteristics would surge or perform inefficiently at some operating points.
B. Description of the Prior Art
One control system for centrifugal liquid chilling machines utilizing adjustable circuitry components is shown in U.S. Pat. No. 3,780,532 to Norbeck et al and assigned to the assignee of the present invention. The control system of Norbeck et al is directed to the elimination of "droop" associated with proportional position control.